1. Field of the Invention
The present invention relates to an ultrasonic ground speedometer utilizing Doppler effect which is, for example, adapted for detection of vehicle speed over the ground. Specificaly to a speedometer which is capable of providing high accuracy of ground speed measurement.
2. Description of the Prior Disclosure
Recently, there have been developed and proposed various ultrasonic ground speedometers. Such ground speedometers generally include an ultrasonic transmitter for outputting an ultrasonic wave having a predetermined frequency, an ultrasonic receiver for receiving the ultrasonic wave when reflected from the road surface and for generating a reflected ultrasonic wave signal, and an arithmetic circuit for deriving ground speed from a Doppler shift occurring in the output ultrasonic wave due to Doppler effect.
One such ultrasonic ground speedometer has been disclosed in Japanese Patent First Publication (Tokkai) Showa 60-76678.
As is generally known, the aforementioned arithmetic circuit derives the ground speed from the Doppler shift according to the following equation: EQU f.sub.d .apprxeq.2f.sub.o.v cos .theta./C
wherein f.sub.d is a Doppler frequency or a Doppler shift, f.sub.o is a basic or output frequency of an output ultrasonic wave emitted from the transmitter, v is a vehicle speed, .theta. is an emitting angle of the output ultrasonic wave relative to the road surface 5 as shown in FIG. 1, and C is the sound velocity of the output ultrasonic wave. In the aforementioned equation, the Doppler frequency f.sub.d is a plus value when the output ultrasonic wave is obliquely emitted in the vehicle forward direction, while the Doppler shift frequency f.sub.d is a minus value when the output ultrasonic wave is obliquely emitted rearward of the vehicle direction.
As appreciated from the above described equation, if the Doppler frequency f.sub.d is derived, the vehicle speed v will be derived, because other parameters f.sub.o, .theta. and C are known quantities. The sound velocity C is generally equal to approximately 340 m/sec (1224 km/h) at ordinary atmospheric temperature. While the parameters f.sub.o and .theta. are respectively fixed to predetermined constant values, the Doppler shift f.sub.d is proportional to the vehicle speed v.
Assuming that the vehicle has a vehicle speed range of 0 to 200 km/h and the emitting angle .theta. is 45.degree., the Doppler shift f.sub.d may vary from 0 (at a vehicle speed of 0) to approximately 0.23 f.sub.o (at a maximum vehicle speed of 200 km/h). The frequency of the reflected ultrasonic wave signal from the ultrasonic receiver (the received frequency) is represented by the sum between the frequencies f.sub.o and f.sub.d. Therefore, assuming that the output frequency f.sub.o of the output ultrasonic wave is 120 kHz, the Doppler shift f.sub.d varies from 0 to a maximum Doppler shift frequency f.sub.dmax of 27.6 kHz (f.sub.dmax =approximately 0.23 f.sub.o) and consequently the received ultrasonic wave signal of the ultrasonic receiver varies from 120 kHz to 147.6 kHz (120 kHz +27.6 kHz).
In the aforementioned conventional ultrasonic ground speedometers, an ultrasonic transmitter includes an echo sounder transmitter traditionally consisting of a piezoelectric echo sounding microphone, while an ultrasonic receiver includes an echo sounder receiver also consisting of a piezoelectric echo sounding microphone. As is well known, since in this type of application echo sounding microphones are arranged in a severe environment, such as an underfloor of a vehicle, a sealed type construction is required. Such a sealed type echo sounding microphone has a particular resonance frequency depending on the geometry of its enclosure and as a result the sealed type or resonance type microphone provides the highest sound pressure sensitivity at its resonance frequency. FIG. 3 is an exemplified characteristic curve illustrating the relationship between the sound-pressure sensitivity and the frequency of the ultrasonic wave received by a traditional resonance type microphone having a resonance frequency of 120 kHz. As clearly seen from the graph of FIG. 3, the microphone exhibits a maximum sound-pressure sensitivity of approximately 110 dB at its resonance frequency. However, if the received ultrasonic wave frequency is other than the resonance point of the microphone, the sensitivity of the microphone becomes drastically lowered. That is, a resonance type microphone has a relatively narrow frequency range with regard to high sensitivity. All resonance type microphones display essentially the same tendency as to sound-pressure characteristics as are seen in FIG. 3. As previously described, since the received frequency varies from the output frequency f.sub.o to the sum (f.sub.o + f.sub.dmax), it is desirable that an echo sounder receiver has high sensitivity over a wide frequency range as described previously. The received frequency (f.sub.o + f.sub.dmax) is representative of a maximum received frequency if the maximum Doppler shift f.sub.dmax is plus, while the received frequency (f.sub.o + f.sub.dmax) is representative of a minimum received frequency if the maximum Doppler shift f.sub.dmax is minus. On the contrary, as appreciated from FIG. 3, since resonance type microphones generally exhibit a relatively narrow frequency characteristic with regard to high sensitivity, high sensitivity coverage over a wide frequency range, of f.sub.o to (f.sub.o + f.sub.dmax), cannot be satisfied. Therefore, even if the output frequency f.sub.o of the transmitter is set to a value suitable to the sensitivity characteristic of a resonance type microphone serving as the ultrasonic receiver, a sufficient S/N (signal-to-noise) ratio of the received frequency signal or the Doppler shift signal cannot be obtained within a wide frequency range, exhibiting low sensitivity, but can be obtained only within the particular narrow frequency range close to the resonance frequency of the microphone.
The aforementioned problem of resonance type microphones will be hereinbelow detailed according to the frequency/sensitivity characteristic curves of FIGS. 4 and 5.
FIG. 4 is a frequency/sensitivity characteristic curve of a resonance type microphone having a resonance frequency of approximately 134 kHz.
Referring now to FIG. 4, the output frequency f.sub.o is set to a particular value such that the resonance frequency of the ultrasonic receiver becomes a middle value of f.sub.o + f.sub.dmax /2 between the output frequency f.sub.o and the maximum or minimum received frequency (f.sub.o + f.sub.dmax) so as to provide a relatively high sound-pressure sensitivity over the widest possible frequency range. Assuming that a maximum vehicle speed and an emitting angle of the output ultrasonic wave are respectively set to 200 km/h and 45.degree., the output frequency f.sub.o may be selected at a particular frequency of for example 120 kHz to satisfy the above mentioned condition. In this manner, if the output frequency f.sub.o is fixed to 120 kHz, the received frequency of the ultrasonic receiver varies within a frequency range of 120 kHz (at a vehicle speed of 0) to 147.6 kHz (at a maximum vehicle speed of 200 km/h). The resonance type microphone therefore exhibits the highest sensitivity, approximately 107 dB (at a middle vehicle speed of 100 km/h), at its resonance frequency, which is set to an essentially middle value between the output frequency 120 kHz and the maximum received frequency 147.6 kHz (corresponding to the sum of the output frequency f.sub.o and the maximum Doppler shift f.sub.dmax). By selecting an optimum value of the output frequency f.sub.o, a possible high sensitivity is obtained for the previously described resonance type microphones. However, even if the output frequency f.sub.o is set to an optimum value depending on the frequency/sensitivity characteristic of the resonance type microphone, the microphone exhibits insufficient low frequency sensitivity (less than 90 dB) at frequency ranges corresponding to the received frequencies at vehicle speeds 0 and 200 km/h.
As set forth above, in resonance type microphones, sufficient sensitivity is not obtained at frequencies other than the resonance frequency, and as a result a sufficient S/N ratio of the reflected ultrasonic wave signal from the ultrasonic receiver or the Doppler shift signal from the arithmetic circuit is not obtained. This results in low accuracy of ground speed measurement.
FIG. 5 shows another type piezoelectric microphone serving as an ultrasonic receiver. This microphone has relatively flat frequency/sensitivity characteristics. As will be appreciated by comparing the frequency/sensitivity characteristic curves shown in FIGS. 4 and 5, the absolute sound-pressure sensitivity of the microphone of FIG. 5 is considerably less than that of FIG. 4 at all frequency ranges. In other words, the microphone shown in FIG. 5 is so designed that frequency/sensitivity characteristics become flat at the sacrifice of absolute sound-pressure sensitivity at its resonance point and in the vicinity of the resonance point. Therefore, such microphones with flat frequency/sensitivity characteristics cannot provide a high enough S/N ratio of received ultrasonic wave signals due to low sensitivity throughout its frequency range.